据说要把固件升级到1.0.5就好了。你的固件的版本是什么？
另外的一个解释是，当你用这几档的时候，对焦点变成了一个，而且是在最下方，这时你要手工设定对焦点（多点对焦），设定以后就可以正常工作了。
下面是一段英文的解释：
If, in Av you press the * button (I
think - or the one next to it on the far right) you can then use the
dial near the shutter button to select the focus point you want to use.
As you turn the dial different focus points will light up in red. I'm
guessing you had it on 'auto' (all focus points light up as you turn the
dial) before, and somehow selected just one of the outer points. Note
that this definitely also works in Tv and M modes and I think in P mode
and that the setting will be stored in memory even if you turn off the
camera. Thus if you change it, the camera won't aromatically go back
to the default of using all focus points when you turn it off and on
again.
It is nearly always better to choose one focus point (the center is the
best), place that were you want to focus, lock focus and recompose. You
may be asking what that all means, I suggest a search, but one way of
locking focus (the default) is to 1/2 press the shutter button and keep
it pressed while you move the camera to the composition you want.
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